In an image-sensing system of a camera lens, etc., it is a frequently employed technique to combine a lens formed of a high-dispersion glass and a lens formed of a low-dispersion glass in order to correct chromatic aberration. JP54-90218A discloses one example of the above low-dispersion glass.
Meanwhile, for correcting chromatic aberration by a compacter constitution, it is desirable to increase a difference between the Abbe's number of a high-dispersion glass and that of a low-dispersion glass and at the same time increase the refractive index of the low-dispersion glass while. When the refractive index is increased, there can be produced a lens that has a desired refractive index and that simultaneously has the curvature of its optical-function surface decreased.
For achieving the above object, there is desired an optical glass having a refractive index nd of 1.86 or more and an Abbe's number νd of 38 or more.
However, a glass that satisfies the above optical properties has a large content of rare earth metal component(s), and the content of component(s) for forming a glass network is relatively decreased, so that it has a problem that it is degraded in devitrification resistance and is easily devitrified when shaped by softening it under heat.
In the invention disclosed in the above laid-open patent Publication, a large amount of Ta2O5 is incorporated for overcoming the above problem. Since, however, Ta2O5 is rare and expensive, a production cost is increased. As a result, the problem is that the use of the above glass is limited although it is a useful glass.